1. Field of the Invention
The present invention generally relates to electromagnetic (e.g. radio) signal detection and processing and, more particularly, to correction for unavoidable differences between electronic components in separate analog channels for processing in-phase (I) and quadrature (Q) portions of the detected signal.
2. Description of the Prior Art
There are numerous applications, including passive ranging and radar applications as well as radio communications in which it is desirable to detect a signal of a frequency which is not initially known but which lies within a given frequency band. For such detection, analog filtering in accordance with the given frequency band is, of course, well-known. However, at the present state of the art, digital signal processing for filtering is also well-known and generally preferred, where feasible, in view of the relative flexibility of modification of the processing by suitable programming of a digital data processor and the precision with which digitized signals can be processed.
Unfortunately, for digital signal detection over a frequency band by digital processing, a wide frequency band for detection requires a correspondingly high sampling frequency. A high sampling frequency increases the amount of data which must be digitally processed and increases hardware overhead, cost, size and weight while potentially compromising response time, depending on available data processor capacity or power. Therefore, as a practical matter, analog signal processing for signal detection and filtering may remain the technique of choice for a given application or device design, particularly when the device must monitor a broad frequency band. It should also be appreciated that even when analog signal processing is utilized for detection, filtering and/or any other desired processing, conversion of the data to digital form for additional processing may be done at any point at which digital processing becomes relatively more desirable or convenient.
In many of the applications alluded to above, the received signal is often converted into an in-phase (I) component and a quadrature (Q) component which are simultaneously processed in parallel in respective signal channels. When analog signal processing circuits are employed to generate the I and Q signals, unavoidable variations between individual electrical components in the respective channels, in the aggregate, cause variations in response between the channels (thus referred to as I/Q imbalances) which are frequency dependent and largely unpredictable.
As is known, such imbalances may take the form of amplitude imbalance, phase imbalance and/or DC offset. DC offset is manifested in the production of an image signal at the center of the band of interest and can thus be corrected or compensated with relative ease since the band of interest will define the center frequency. However, combinations of amplitude and phase imbalance are manifested as an image frequency reflected about the center frequency of the frequency band from the received signal, the frequency of which is not necessarily known.
The amplitude of the image signal is a function of the magnitude of the amplitude and phase imbalances and, although it will be less than the amplitude of the received signal, may be significant. Both amplitude and phase imbalance are frequency dependent and, while correction values for correcting such imbalances can be determined by careful calibration of the detector, receiver or other circuit including the parallel, analog I and Q channels, the proper correction values cannot be chosen and applied without knowledge of the frequency of the received signal. If the I and Q data is digitized and subsequently processed without correction or compensation for the image signal, the image signal data will be included in the digitized representation thereof and thereafter will be inseparable from the received signal. The image signal can thus degrade the performance of algorithms used to estimate the parameters of the received signal, appear as an additional signal which requires separate processing and/or mask the existence of a weaker signal which may also be of interest.
Techniques are known for correction of I/Q imbalances using digital signal processing techniques applied to digitized versions of the I and Q signals such as that described in "The Correction of I and Q Errors in a Coherent Processor" By Churchill et al., IEEE Trans. Aerospace and Electronics, January, 1981, pp. 131-137 and U.S. Pat. No. 3,950,750. An improved implementation of the technique disclosed by Churchill et al. is disclosed in U.S. Pat. No. 5,105,195 to Conrad in which a known test signal is periodically applied for calibration and computation of correction factors which are fed back to a compensation circuit.
However, these techniques are applied in the time domain and, since the correction to be applied is frequency dependent, some additional technique of determining the frequency of the received signal is needed but may not be readily available, as in the applications alluded to above where the frequency of the received signal is initially unknown. In addition, the implementation of Conrad does not provide the ability to compensate at a plurality of frequencies, as is essential in many application areas.
To obtain frequency information concerning the detected signal, it is possible to compute the discrete Fourier transform (DFT) of the signal by performing a fast Fourier transform (FFT), determining the frequency of the dominant signal and then applying the appropriate frequency-dependent corrections to the time-domain signal. However, if further frequency domain processing is to be done on the corrected signal, as is often desirable, a further FFT must be performed on the corrected time domain signal to obtain a representation of the signal in the frequency domain for such further processing. Thus, two FFTs are required to achieve frequency-dependent correction for I/Q imbalance by the known techniques described above and, thereafter, to allow further frequency domain processing of the signal to be accomplished.
For example, the detection of multiple continuous wave (CW) signals and correlation processing for frequency and phase estimation which is often encountered in position locating or ranging systems requires additional frequency domain processing and possibly further I/Q imbalance correction for additional detected frequencies. Such further I/Q imbalance correction by the above-described techniques would require further pairs of FFTs to be performed which may be beyond the capacity of special purpose FFT processors which are otherwise feasible for use in a particular application, particularly if a high sampling rate must be provided by the design.
An early attempt to provide frequency domain I/Q imbalance correction is disclosed in U.S. Pat. No. 4,003,054 to Goldstone. However, the approach disclosed therein requires the application of a plurality of linear chirp signals, each covering a different frequency band to compute a set of frequency-dependent correction factors equally spaced across the frequency band of interest. As with the time domain correction of Conrad, the requirement for a particular calibration signal to be applied does not provide a generalized system for correction. Linear chirp signals, in particular, while used in modern radar systems, are not available in many applications for which I/Q imbalance correction may be desired.
Further, Goldstone applies all of the correction factors to the FFT of a received signal which can be computationally intensive when processing using large FFT sizes and does not assure that the frequencies for which correction factors are available closely coincide with the frequency of the received signal of interest. That is, the development of equally spaced correction factors across the frequency band of interest is a constraint which limits optimization of performance in regard to an arbitrary frequency of a signal for a given implementation complexity. In addition, when the signal is, for example, a narrowband CW signal concentrated in a very small region of the entire frequency band being processed, applying correction factors over the entire frequency band (as is done in Goldstone) can degrade the performance of subsequent processing.
Accordingly, there is a need for a technique of I/Q imbalance correction employing simplified calibration signals which can be carried out in the frequency domain when the frequency of the received signal is not initially known, and is well-suited to processing narrowband CW signals.